A communication network system in which a management server performs centralized management of a communication network including a plurality of nodes is known. The management server, when receiving a route setup request regarding a certain flow, sets up a communication route of the flow in the communication network. Such the management server that sets up the communication route of the flow is hereinafter also referred to as a “route setup server”. Techniques regarding the route setup server are described, for example, in Patent Literature 1 (Japanese Patent Publication JP-2008-131346) and Non-Patent Literature 1 (Martin Casado et al., “Ethane: Taking Control of the Enterprise”, In Proceedings of SIGCOMM 2007.).
For example, a certain terminal starts transmission of packets of a new flow. When a first packet arrives at a first node, the first node does not know a forwarding destination of the received packet. Therefore, the first node transmits the received packet as a route setup request to a route setup server. In response to the route setup request, the route setup server determines a transfer route of the packets of the new flow. Then, the route setup server instructs each node on the determined transfer route to forward the packets along the determined transfer route. As a result, each node becomes able to forward packets of the same flow along the designated transfer route. Thereafter, communication of the flow is performed without through the route setup server.
Here, let us consider a case of performing the route setup for a flow extending across a plurality of subnets. In this case, not only the route setup within each subnet but also the route setup between subnets needs to be performed. For example, in a case of a network composed of Layer 2 and IP, routing within each subnet is performed by the Layer 2 and routing between subnets is performed by the IP.
Non-Patent Literature 2    (http://www.cse.wustl.edu/˜7Ejain/atm/atm_mpoa.htm, “Multiprotocol over ATM (MPOA)”) describes an MPOA (Multi-Protocol Over ATM) technique. According to the MPOA technique, the route setup request namely the first packet of a communication flow is actually communicated between routers. When the packet arrives at an exit of the network, a route of the flow is set up based on the route through which the packet actually has gone.